For many decades the taxonomy of the genus Dinlostomum von Nordmann, 1832 has remained in a state of confusion in the literature. The present study aimed at determining where the problems lie and what-can be done to resolve them. A brief survey of the farmed and wild populations of fish in Scotland indicated that there were four highly prevalent forms of Diolostomum metacercariae in the eyes of the fish. Type 1 were located in the lens of rainbow trout (Salmo gairdneri Richardson), Type 2 in the humour of rainbow trout, Type 3-in the retina of rainbow trout and Type 4, in the retina of perch (Perca fluviatilis Mitchell). The four types were examined morphmetrically using-Principal Components Analysis (PCA) and this revealed that the four types grouped separately. Type 1 metacercariae could be distinguished by the closeness of their lappets, Type 3 by their small size, Type 4 by their large size and high breadth to length ratio and Type 2 lacked any of these distinguishing features. The culture of the metacercariae to adults revealed biological differences between the four types. Type 1 grew in both domestic chickens and herring gull chicks (Larus argentus Pontoppidan), but best in the latter. Type 2 also grew in both of these hosts, but best growth was achieved in the chicken. Chickens were refractive to Types 3 and 4, but both established and developed in herring gulls. The adults obtained from the bird host were also examined morphometrically using PCA, and again the four forms grouped separately, indicating that they were morphologically distinct. In experimental infections it was found that the hind-body did not fully develop until day 16-18 p. 1. and, therefore, morphological analysis was only carried out on worms which had been cultured in the bird host for more than 16 days. This was particularly significant in Type 1 adults where the relative position of the ovary was affected. Type 1 adults were distinguished from the others by the posterior position of the ovary in the hind-body, Type 2 by the small dimensions of the ovary and its position at the intersegmental region, Type 4 by the anterior extent of the vitellarlum and Type 3 adults lacked any of the distinguishing features. Completion of the life-cycle of the diplostomes also revealed differences in the cercarial stage both morphologically and in the sensory papillar patterns determined by the use of chaetotaxy. The life-cycle was completed for all the diplostomes apart from Type 2. It was found that Type 2 miracidia would not establish in Lvmnaea Dereger, although the other three types established. Infection of fingerling rainbow trout with Types 1,3 and 4 cercariae revealed that the trout were refractive to Type 4 cercariae, but Types 1 and 3 established in the lens and retina, respectively. When perch were exposed to Type 4 cercariae it was found that metacercariae established in the retina. This indicated that the metacercariae are very site specific and also may show some host specificity. Biochemical analysis of the metacercarlae of Types 1-4 by analysing isoenzyme profiles with the use of Isoelectric Focusing (IEF) also revealed that there may also be some differences biochemically between the four types. Identification of the four diplostomes was attempted using the keys (Dubois, 1970; Shigin, 1986) and information available in the literature. However, a confident identification could only be made for Type 1 which keyed down to D. soathaceum sensu Niewiadomska. The other three types could not, however, be identified with such confidence, but tentative identifications were made. The fact that the metacercarial, cercarial and adult stages were obtained and described for three out of the four diplostomes and still a positive identification could not be made perhaps reflects the inadequecies of the keys available in the literature. The present study has successfully determined the problematic areas surrounding the taxonomy of the genus and has, therefore, cleared the way for future study.